一. 概述
当不需要通过send broadcasts来完成跨应用的通信，那么建议采用LocalBroadcastManager， 将会是更加地高效、安全地方式，并且对系统带来的影响也是更小。
- BroadcastReceiver采用的binder方式实现跨进程间的通信；
- LocalBroadcastManager使用Handler通信机制。
1.1 使用实例
//1. 自定义广播接收者
public class LocalReceiver extends BroadcastReceiver {
public void onReceive(Context context, Intent intent) {
...
}
}
LocalReceiver localReceiver = new LocalReceiver();
//2. 注册广播
LocalBroadcastManager.getInstance(context)
.registerReceiver(localReceiver, new IntentFilter(“gityuan”));
//4. 发送广播
LocalBroadcastManager.getInstance(context).sendBroadcast(new Intent("gityuan"));
//5. 取消注册广播
LocalBroadcastManager.getInstance(context).unregisterReceiver(localReceiver);
LocalBroadcastManager注册只能通过代码注册方式，而不能通过xml中注册静态广播，本地广播并不会跨进程通信，不用 跟system_server交互，即可完成广播全过程。
二. 原理分析
2.1 LocalBroadcastManager
[-> LocalBroadcastManager.java]
public final class LocalBroadcastManager {
public static LocalBroadcastManager getInstance(Context context) {
synchronized (mLock) {
if (mInstance == null) {
//单例模式，创建LocalBroadcastManager对象
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
private LocalBroadcastManager(Context context) {
mAppContext = context;
//创建运行在主线程的Handler
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();
break;
default:
super.handleMessage(msg);
}
}
};
}
}
2.2 registerReceiver
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//创建ReceiverRecord对象
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//查询或创建数据到mReceivers
ArrayList<IntentFilter> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<IntentFilter>(1);
mReceivers.put(receiver, filters);
}
filters.add(filter);
//查询或创建数据到mActions
for (int i=0; i<filter.countActions(); i++) {
String action = filter.getAction(i);
ArrayList<ReceiverRecord> entries = mActions.get(action);
if (entries == null) {
entries = new ArrayList<ReceiverRecord>(1);
mActions.put(action, entries);
}
entries.add(entry);
}
}
}
注册过程，主要是向mReceivers和mActions添加相应数据：
- mReceivers：数据类型为HashMap<BroadcastReceiver, ArrayList
>， 记录广播接收者与IntentFilter列表的对应关系； - mActions：数据类型为HashMap<String, ArrayList
>， 记录action与广播接收者的对应关系。
2.3 sendBroadcast
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
final String action = intent.getAction();
final String type = intent.resolveTypeIfNeeded(
mAppContext.getContentResolver());
final Uri data = intent.getData();
final String scheme = intent.getScheme();
final Set<String> categories = intent.getCategories();
//根据Intent的action来查询相应的广播接收者列表
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
if (entries != null) {
ArrayList<ReceiverRecord> receivers = null;
for (int i=0; i<entries.size(); i++) {
ReceiverRecord receiver = entries.get(i);
if (receiver.broadcasting) {
continue; //正在处理广播的receiver，则跳过
}
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
if (match >= 0) {
if (receivers == null) {
receivers = new ArrayList<ReceiverRecord>();
}
receivers.add(receiver);
receiver.broadcasting = true;
}
}
if (receivers != null) {
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
//创建相应广播，添加到mPendingBroadcasts队列
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
//发送消息【见小节2.3.1】
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
主要功能：
- 根据Intent的action来查询相应的广播接收者列表；
- 创建相应广播，添加到mPendingBroadcasts队列；
- 发送MSG_EXEC_PENDING_BROADCASTS消息。
2.3.1 executePendingBroadcasts
private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
//将mPendingBroadcasts保存到brs数组
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);
mPendingBroadcasts.clear();
}
//回调相应广播接收者的onReceive方法。
for (int i=0; i<brs.length; i++) {
BroadcastRecord br = brs[i];
for (int j=0; j<br.receivers.size(); j++) {
br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
}
}
}
}
该过程运行在app进程的主线程，所以onReceive()方法不能有耗时操作。
2.4 unregisterReceiver
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
//从mReceivers移除该广播接收者
ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
//从mActions移除接收者为空的action
for (int i=0; i<filters.size(); i++) {
IntentFilter filter = filters.get(i);
for (int j=0; j<filter.countActions(); j++) {
String action = filter.getAction(j);
ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=0; k<receivers.size(); k++) {
if (receivers.get(k).receiver == receiver) {
receivers.remove(k);
k--;
}
}
if (receivers.size() <= 0) {
mActions.remove(action);
}
}
}
}
}
}
三. 总结
广播注册/发送/取消注册过程都使用同步锁mReceivers来保护，从而保证进程内的多线程访问安全。 最后，再来看看使用LocalBroadcastManager的优势：
- 发送的广播不会离开当前所在的app, 因此不用担心私有隐私数据的泄漏，确保安全性；
- 其他应用也无法发送这类广播到当前app，则不用担心会暴露漏洞而被其他app所利用；
- 比系统全局的广播更加高效，且对系统整体性能影响更小；
那么不足是什么呢？LocalBroadcastManager只能用于同一个进程不同线程间通信，而不能跨进程， 即便是同一个apk里面的两个进程也不行，需要使用BroadcastReceiver。
LocalBroadcastManager与BroadcastReceiver使用场景：
- 同一个进程的多线程间通信，强烈建议使用LocalBroadcastManager；
- 跨进程的通信，则使用BroadcastReceiver。（当然跨进程实现方式有多种，这里只说广播相关的方案）
微信公众号 Gityuan | 微博 weibo.com/gityuan | 博客 留言区交流